Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

045
FXUS63 KLOT 160734
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
234 AM CDT Tue Jun 16 2026

.KEY MESSAGES...

- Period or two of scattered showers and isolated thunderstorms
  expected today, with a level 1 of 5 severe risk east of I-55
  including NW Indiana.

- A dangerous severe weather outbreak, including the potential
  for long tracked, strong-violent tornadoes, is possible
  Wednesday, especially south of I-80.

- Locally heavy rainfall Wednesday could result areas flash
  flooding and potentially some river flooding.

&&

.DISCUSSION...
Issued at 234 AM CDT Tue Jun 16 2026

Today:

Water vapor imagery early this morning depicts a vigorous
shortwave trough digging southeast into the mid-upper
Mississippi Valley with an embedded compact mid-upper level low.
At the sfc, low pressure near Duluth early this morning will
deepen as it moves southeast to near Green Bay by early this
afternoon. A trailing cold front will sweep across the area
later this morning through mid afternoon. While southerly winds
ahead of this front will draw some moisture northward, an
elongated east-west oriented ridge of high pressure to our south
will block the return of any meaningful Gulf moisture today.

A lead band of showers and isolated thunderstorms is expected to
blossom over northern IL toward sunrise as the increased ascent
associated with a lead impulse encounters the better (though
still somewhat meager by mid-June standards) moisture over the
area. This lead band of showers and isolated t-storms should
sweep quickly across the CWA between 11-16z. Despite the limited
moisture, steep lapse rate and cold mid-upper level temps will
likely allow for a narrow axis of up to around 1000 J/kg of
MLCAPE to develop in the wake of the lead band of showers and in
advance of approaching cold front.

Scattered thunderstorms are expected to begin to develop along
and ahead of this front early-mid afternoon, with the big
question for our area being how far east the front gets before
storms develop. Strong shear and moderate instability would
likely support a risk of supercells capable of large hail and
perhaps locally damaging winds. At this point, it appears the
best chance of storms along the cold front in our CWA will be
roughly near/east of I-55, especially across northwest Indiana
where frontal passage will be latest to occur.

Behind the front, look for gusty westerly winds, probably
peaking in the 30-35 mph range at times this afternoon before
quickly subsiding toward sunset.

Wednesday:

An unusually powerful shortwave trough is progged to race
east-southeastward into the upper Midwest and western Great
Lakes Wednesday into Wednesday evening. Model guidance is in
good agreement on the synoptic scale with 500 mb heights
progged to be 3-4 standard deviations below normal for mid June.
At the sfc, an associated low pressure that could threat all-
time monthly record for June is expected to develop across
Wisconsin and eventually move into lower Michigan.
Unsurprisingly, the kinematic fields with such an anomalously
strong system are also nearing the upper echelons of what we
see this time of year in the Midwest. All of this to say that
synoptically, the ingredients are coming together for a
potentially dangerous outbreak of severe thunderstorms and
potentially long-tracked, intense tornadoes.

On the mesoscale, there are naturally greater uncertainties
regarding precisely where (and to a lesser extent, if) the
overlap of exceptionally strong deep and low level shear and
moderately strong instability will take place. SPC`s latest
SWODY2 highlights areas south of I-80 in our CWA with a rare
day 2 moderate risk (level 4 of 5) for Wednesday. The potential
ceiling for bad this severe weather event could get is quite
high, but it is also important to note there are obviously still
failure modes that could materialize on the mesoscale and
prevent the reasonable worst case scenario from unfolding.

The general expectation as it looks now is that a strong low
level jet (increasing to 60kt+ at 850 mb by 12z Wed) will result
in very strong low level theta-e advection and eventually the
development of strong to severe thunderstorms late tonight.
Initially development is expected over eastern IA, but as the
low level jet translates eastward into IL, it should support
this convection developing/moving into northern IL near/after
sunrise Wednesday morning. While this convection will be
elevated, strong effective shear and a reservoir of 1000-2000
J/kg of MUCAPE should allow for elevated supercells. The
greatest severe risk Wednesday morning looks to be over our
western and eventually southern CWA. In addition to the threat
large hail, strong shearing instability near a sharpening
frontal inversion would point to at least some threat for the
development of gravity wave associated convection. Should this
occur, a damaging wind threat could also develop, despite little
or no sfc based instability. In fact, there could even be
pockets of locally significant severe wind gusts (>75 mph) near
or just west/south of our CWA Wednesday morning. This
convection will probably evolve into an MCS as it tracks east
across northern IL and into northwest Indiana Wednesday morning.

The morning convection will likely augment the warm frontal
position and at least initially slow the northward progression
of the composite warm front/outflow boundary, delaying
destabilization north into our CWA. This seems to be the most
obvious potential failure mode: morning convection retarding
the northward surge of the warm front and subsequently the
stronger instability, keeping the extreme wind shear profiles
and strong synoptic forcing somewhat divorced from the more
favorable instability. While that is one potential obvious
failure mode, at this point, it seems unlikely to fully succeed
in completely disrupting the otherwise exceptionally favorable
synoptic set-up from resulting in a high end severe threat.
Though the precise location of the most likely area(s) to see a
high end event, could change some on the mesoscale as the event
nears.

The reason that this morning convection is unlikely to
completely stunt the northward surging warm front is the
extremely strong mass response expected as a result of the near
record deep low pressure system. With a far less intense low
last Thursday, an impressively large footprint of a cold pool
left behind from an MCS that lingered well into the afternoon
was able to be completely overcome in just a matter of a couple
of hours late in the day. Something similar seems plausible
again tomorrow afternoon where a dissipating outflow boundary
from early convection could separate an extremely volatile air
mass from a still sufficiently unstable air mass north of the
boundary could support severe thunderstorms with damaging winds
and isolated tornadoes.

There is variance in guidance with just how far north the
effective boundary will get, but somewhere generally in the
vicinity of I-80 is where a majority of the models show it
reaching its peak latitude in our CWA. To the south of this
boundary, the environment looks similar to what`s often seen in
the cool season major tornado outbreak in the southeastern
United States. Forecast hodographs are literally off the chart,
with 0-1 environmental SRH >500 m^2/s^2. This sort of extreme
low level shear, coupled with low LCLs and resultant strong
0-3km CAPE, fast storm motion, and favorable downstream
environment is the classic type of set-up long tracked strong-
violent tornadoes.

It`s important stress that while the synoptic set-up is classic
for a tornado outbreak, we are dealing with convection that
often alters the mesoscale environment in ways that cannot be
anticipated 12-24+ hours in advance. As noted above, there are
certainly scenarios where an alteration of the mesoscale
environment could dramatically reduce or shift the area of the
greatest tornado risk. For this reason, it is important to
monitor later forecasts closely. Finally, in addition to the
supercell tornado threat, a fast moving line of severe
thunderstorms/possible derecho may also accompany the front
producing widespread, potentially significant, wind damage as
well as line embedded tornadoes, some possibly strong.

Not to be overlooked, precipitable water values are progged to
get to near or just above 2", meaning that convection will
likely be extremely efficient rain producers. The stronger
convection could easily produce 2"+ of rain in just an hour or
so. Given the antecedent very moist ground and generally above
average streamflow, there is a threat of flash flooding
Wednesday. The greatest threat may be with the first round of
convection, since it will likely be a bit slower moving and
offer a better chance of some training cells than the second
round. When the area most at risk can be better refined, we will
need a flood watch for portions of the area for Wednesday as
well.

After Wednesday:

We should see a break in the active pattern for the rest of the
work week. There are indications that we could get into a more
active northwest flow type pattern heading into the weekend and
beyond.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1230 AM CDT Tue Jun 16 2026

Forecast concerns include...

Chance of thunderstorms this morning and again this afternoon.
Gusty southwest winds this morning, then west/northwest this
afternoon.

Overall, current forecast is in good shape with only some timing
thunder tweaks this afternoon. A broken line of showers and
scattered thunderstorms is expected to move across the terminals
this morning. Current tempo mention has this potential handled
well, though duration at any one location may end up being 1-2
hours. A second line of scattered thunderstorms is expected to
develop either right over the Chicago terminals or just east of
the Chicago terminals during the early/mid afternoon hours.
Adjusted current prob thunder an hour later but duration during
this time may be 1-2 hours as well. A few lingering showers may
be possible across far northeast IL/southwest WI and over Lake
Michigan through early this evening.

Southwest winds around 10kt overnight will slowly increase
toward daybreak when gusts to 20kt will be possible. Winds/gusts
will continue to increase through the late morning and then turn
west/northwest this afternoon behind the cold front. Gusts into
the upper 20kt range are expected. Speeds and gusts will
diminish quickly with sunset this evening.

Some mvfr level cigs are possible this morning and again this
afternoon, but will most likely be associated with precipitation
and not expecting prevailing mvfr cigs. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 9 AM this morning to 9 PM CDT this
     evening for the IL and IN nearshore waters.

&&

$$

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